Small scale statistics of viscoelastic turbulence

The small scale statistics of homogeneous isotropic turbulence of dilute polymer solutions is investigated by means of direct numerical simulations of a simplified viscoelastic fluid model. It is found that polymers only partially suppress the turbulent cascade below the Lumley scale, leaving a remnant energy flux even for large elasticity. As a consequence, fluid acceleration in viscoelastic flows is reduced with respect to Newtonian turbulence, whereas its rescaled probability density is left unchanged. At large scales the velocity field is found to be unaffected by the presence of polymers.Comment: 7 pages, 4 figure

Lagrangian statistics in two-dimensional free turbulent convection

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Nonlinear dynamics of the viscoelastic Kolmogorov flow

The weakly nonlinear regime of a viscoelastic Navier--Stokes fluid is investigated. For the purely hydrodynamic case, it is known that large-scale perturbations tend to the minima of a Ginzburg-Landau free-energy functional with a double-well (fourth-order) potential. The dynamics of the relaxation process is ruled by a one-dimensional Cahn--Hilliard equation that dictates the hyperbolic tangent profiles of kink-antikink structures and their mutual interactions. For the viscoelastic case, we found that the dynamics still admits a formulation in terms of a Ginzburg--Landau free-energy functional. For sufficiently small elasticities, the phenomenology is very similar to the purely hydrodynamic case: the free-energy functional is still a fourth-order potential and slightly perturbed kink-antikink structures hold. For sufficiently large elasticities, a critical point sets in: the fourth-order term changes sign and the next-order nonlinearity must be taken into account. Despite the double-well structure of the potential, the one-dimensional nature of the problem makes the dynamics sensitive to the details of the potential. We analysed the interactions among these generalized kink-antikink structures, demonstrating their role in a new, elastic instability. Finally, consequences for the problem of polymer drag reduction are presented.Comment: 26 pages, 17 figures, submitted to The Journal of Fluid Mechanic

Two-dimensional elastic turbulence

We investigate the effect of polymer additives on a two-dimensional Kolmogorov flow at very low Reynolds numbers by direct numerical simulations of the Oldroyd-B viscoelastic model. We find that above the elastic instability threshold the flow develops the elastic turbulence regime recently observed in experiments. We observe that both the turbulent drag and the Lyapunov exponent increase with Weissenberg, indicating the presence of a disordered, turbulent-like mixing flow. The energy spectrum develops a power-law scaling range with an exponent close to the experimental and theoretical expectations

Lagrangian velocity structure functions in Bolgiano turbulence

Single-particle Lagrangian velocity statistics in the Bolgiano–Obukhov regime of two-dimensional turbulent convection is investigated. At variance with flows displaying the classical K41 phenomenology, here the leading contribution to the Lagrangian velocity fluctuations is given by the largest eddies. This implies a linear behavior in time for a typical velocity fluctuation in the time interval t. The contribution to the Lagrangian velocity fluctuations of local eddies (i.e., with a characteristic time of order t), whose space/time scalings are ruled by the Bolgiano–Obukhov theory, is thus not detectable by standard Lagrangian statistical observables. To disentangle contributions arising from the large eddies from those of local eddies, a strategy based on exit-time statistics has successfully been exploited. Lagrangian velocity increments in Bolgiano convection thus provide a physically relevant example of a signal with more than smooth fluctuations

Being positive. Strategie e linguaggi per la comunicazione dell'HIV

Il libro raccoglie i contributi critici di alcuni studiosi da tempo impegnati nella ricerca teorico-applicativa sulla rappresentazione per la comunicazione sociale, mettendo in evidenza i tratti precipui dei linguaggi visivi prevalenti. Lo studio verte, in particolare, sull'analisi delle retoriche e dei segni utili a veicolare i messaggi in differenti tipologie di prodotti e canali: dal disegno alla grafica, all'audiovisivo, al web, con riferimento a utenze distinte in termini anagrafici

Two-dimensional elastic turbulence

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